validation: use noexcept instead of deprecated throw()

```bash
  CXX      libbitcoin_node_a-validation.o
validation.cpp:5164:30: warning: dynamic exception specifications are deprecated [-Wdeprecated-dynamic-exception-spec]
    const char* what() const throw() override
                             ^~~~~~~
validation.cpp:5164:30: note: use 'noexcept' instead
    const char* what() const throw() override
                             ^~~~~~~
                             noexcept
```

src/validation.cpp

@@ -5161,7 +5161,7 @@ static void FlushSnapshotToDisk(CCoinsViewCache& coins_cache, bool snapshot_load
 
 struct StopHashingException : public std::exception
 {
-    const char* what() const throw() override
+    const char* what() const noexcept override
     {
         return "ComputeUTXOStats interrupted.";
     }